1. Field of the Invention
The present invention relates generally to portable chain saw sharpening devices, in which the sharpener is preferably adapted to be removably mounted to a bracket disposed on the chain saw. In particular, the present invention is directed to a bottom sharpening chain saw sharpener, in which the bottom surfaces of the cutting teeth of a conventional bottom sharpened saw chain are conveniently sharpened without disturbing the top surfaces of the saw chain elements, including the depth gauge lifting member disposed forward of each cutting tooth. The saw chain is sharpened by running the chain under power through the sharpener so as to sharpen the cutting teeth of the saw chain quickly, preferably in a matter of twenty to thirty seconds, without removing the chain from the chain bar.
2. Description of the Prior Art
Maintaining the sharpness of a chain saw is particularly important for maintaining cutting efficiency and for preventing injuries which can result from snagging the chain as a result of dull cutting teeth. It is well known that a saw chain can become dull quite quickly when the chain saw is used for moderate to heavy jobs, and it is not uncommon for the chain in heavy use to require sharpening twice in one day. Therefore, a method and apparatus for sharpening a saw chain quickly and conveniently is important to minimize down time on any particular job. It is preferable that the saw chain can be sharpened without removing the chain from the saw, in order to reduce the time and trouble necessary to sharpen the chain. In addition, it is desirable for the sharpener to be easily portable, so that the sharpener can be brought along to remote jobs where a chain saw is often used. It is also preferable that the cutting teeth of the saw chain are able to be sharpened all at one time, without having to pay individual attention to each of the cutting teeth on the saw chain. It is important for a chain sharpener to be able to bottom sharpen a conventional saw chain, wherein the bottom surfaces of the cutting teeth are ground or honed, so as to avoid the use of top sharpening saw chains, which are rather uncommonly used in commercially available chain saws.
Large industrial sized machines are available for sharpening saw chains, but these devices require removing the chain from the saw and leaving the chain for sharpening by an experienced professional for later pick-up. It is obvious that this method is time consuming and inconvenient, and requires the intensive chain saw user to keep a number of extra chains on hand so that in the field a sharp chain is readily available to replace a dull chain.
A number of devices exist which aid the chain saw user in sharpening the saw chain while the chain remains on the saw. Such devices commonly employ a fixture, which must be clamped over the chain bar to aid in aligning a hand file to the proper pitch for sharpening each of the cutting teeth. U.S. Pat. No. 4,404,871, "Portable Chain Saw Sharpening Kit With Cutting Link Alignment Means and Method for Using Same" to Fritz et al. discloses a chain sharpener in which the cutting surfaces of each of the cutting teeth are individually sharpened using a hand powered rotary burr. The burr is inserted into one of two guide bores, depending upon whether a left or right bottom sharpened cutting tooth is to be sharpened, and the fixture is moved from tooth to tooth as each of the bottom surfaces of the cutting teeth are sharpened. While this device has the advantage that it is simple in construction and is easily portable for use in the field, the device requires a large amount of time and effort to individually sharpen by hand each of 20 to 30 cutting teeth disposed on the chain.
Accordingly, a number of "self-sharpening" chain saws have been developed in which a sharpener is built into the housing of the chain saw such that the chain is either automatically sharpened as the saw is used or the chain is sharpened whenever the user actuates a sharpening lever. However, all of these sharpeners are top sharpening devices in which a special chain having cutting teeth with sharpenable surfaces on a top surface must be used. In the typical self-sharpening device, an abrasive element is disposed in the housing of the chain saw adjacent to the drive sprocket, and the abrasive element is applied to top surfaces of the cutting teeth to grind a new edge as the chain is run under power around the chain bar. Typical devices of this type are disclosed in U.S. Pat. Nos. 3,526,156; 4,506,565, 3,301,098; 3,040,602; and 4,457,069.
A typical self-sharpening device is disclosed in U.S. Pat. No. 3,526,156, "Saw Chain Sharpener Means" by Condon et al., in which an arcuate sharpening stone is disposed in the housing of the chain saw adjacent to the drive sprocket, such that a button positioned on the top of the chain saw housing can be depressed to bring the grinding stone into contact with the top of the chain as the chain moves around the sprocket under power. Naturally, only the top surfaces of the cutting teeth can be sharpened by the sharpening stone, so that such devices are only useful when a top sharpening saw chain is employed on the chain saw. Thus, a saw chain of the type which is ordinarily used on most commercially available chain saws, in which bottom surfaces of the cutting teeth are ground to produce the cutting edge, cannot be used on this type of self-sharpening chain saw. Another problem arises with the self-sharpening chain saw because the grinding wheel soon becomes worn away, resulting in non-uniform sharpening of the saw chain teeth. When this occurs, both the saw chain and the sharpening element must be replaced.
U.S. Pat. No. 3,435,860, "Top Sharpening Floating Sprocket Chain Saw" to Silvon, discloses an arcuately shaped sharpening member having a curvature approximately identical to the curvature of the drive sprocket. The curved sharpening member is disposed in the chain saw housing adjacent the sprocket, and the sharpening member is designed to be urged by an actuator or key to engage the saw chain. The sharpening member can be pressed by the operator against the top surfaces of the cutting teeth at the location of the chain sprocket so as to uniformly grind the top surfaces of the cutting teeth while the chain is powered around the chain bar. In so doing the sharpening member grinds not only the top surfaces of the cutting teeth but also grinds and reduces the height of the depth gauge members disposed just forward of each cutting tooth.
Another type of top sharpening saw chain sharpener is disclosed in U.S. Pat. No. 4,597,178, "Saw Chain Sharpener" to Dolata et al., in which an arcuate grinding element is disposed in the front portion of an elongated chain guard casing which can be removably disposed over the chain bar. In this sharpener, the grinding element is disposed in the cavity at the front of the casing and a knob disposed behind the grinding element is used to press the element onto the top of the chain while the chain is powered around the chain bar. This sharpener is also a top sharpening device which sharpens the top surfaces of the cutting teeth and also reduces the length of the depth gauges, as does the Silvon device. Because this sharpener is not disposed in the chain saw housing, presumably the chain saw can be made lighter than those devices which incorporate the sharpening lifting member and the necessary supporting structure in the housing. However, as with all of the "self sharpening" devices described above, the Dolata et al. sharpener sharpens only the top surfaces of the cutting teeth, and the height of the depth gauge lifting member is reduced at every sharpening.
A detailed disclosure of a top sharpening saw chain is provided in U.S. Pat. No. 3,269,431, "Chain Saw Sharpening Mechanism" to Ehlen, in which a chain saw having an abrasive element disposed in the vicinity of the drive sprocket is brought into contact with the top surfaces of a chain with cutting teeth specially designed to be sharpened from the top. In order to sharpen the cutting teeth so that trailing ends of the teeth are ground to a greater extent than the leading ends, the sprocket in Ehlen is arranged to have cams which cant the teeth as they move around the sprocket, so as to present the proper grinding angle to the sharpening lifting member.
However, none of the devices discussed above are capable of "automatically" sharpening the bottom surfaces of the cutting teeth of the bottom sharpening saw chains which are most commonly used on chain saws commercially available to professionals and hobbyists. Devices previously known in the art in which the bottom surfaces of the cutting teeth can be sharpened have been limited to manual devices in which each tooth is sharpened one at a time by hand while the chain remains on the bar, or automatically by industrial sized devices which sharpen the chain after removal from the chain bar. The difficult problem of sharpening the bottom surface of the cutting teeth while the saw is running using a portable preferably removable sharpener has not been solved by devices previously known in the art.